There are two simple methods. The first is by titration. A precise quantity of the must, usually about 2 ml, is diluted in twice the quantity of distilled water. Six drops of Phenolphthalein are then added as a colour indicator and the whole is gently shaken so that all three are well mixed. A measured quantity of an acid-testing reagent - usually sodium hydroxide - is then added, drop by drop, until the whole solution turns pink.

This is easy to spot with white wines but harder with the very dark reds. If the must is very dark, more distilled water may be added.

When the pink point is reached the quantity of reagent used is noted. The test is then done a second time and if the quantity of reagent used is not precisely the same the average is taken. The number of ml of reagent is then multiplied by 2^ and the answer is the quantity of acid in the must, measured in parts per 1,000 (p.p.t.) of sulphuric acid.

Sulphuric acid is used as a constant with which all other acids are compared.

For example, if titration reveals a sulphuric acid equivalent content of 2.5 p.p.t. and you propose to add citric acid to bring the acidity of your must up to 5 p.p.t. citric acid, you will need to add just under 7 grams (^ oz) of citric acid per demijohn.

It will be seen from table 14 section A that the equivalent p.p.t. of sulphuric acid for citric acid is 1.09 equal to 7 grams (^ oz). Add 1.09 to the 2.50 p.p.t. already recorded from titration and the total is 3.59. Reference to section B again shows that 3.5 p.p.t. sulphuric is equivalent to approximately 5 p.p.t. citric acid. This is much less complicated in practice than it seems in theory.

An over-acid must may be further diluted but precipitated chalk, calcium carbonate, is usually added, especially in a rhubarb must, which may contain some oxalic acid. 7 grams ( oz) chalk in a demijohn of must will reduce the acidity by 1.5 p.p.t. sulphuric acid.

It is interesting to note some typical acidities of certain well-known commercial wines.